1. Field of the Invention
The present invention relates to a non-aqueous electrolyte battery incorporating a positive-electrode active material constituted by a composite lithium oxide.
2. Description of the Related Art
In recent years, considerable progress of a variety of electronic apparatuses has been made. Therefore, rechargeable secondary batteries have been studied which can conveniently be used for a long time with low cost. Representative secondary batteries are exemplified by a lead battery, an alkali battery and a lithium secondary battery. In particular, the lithium secondary battery has advantages of high output and a high energy density. The lithium secondary battery incorporates positive and negative electrodes, to which lithium ions can reversibly be doped/dedoped, and non-aqueous electrolytic solution.
The capacities of the lithium secondary batteries have been enlarged recently. On the other hand, reduction in the cost has been attempted by selecting materials. In particular, a cobalt oxide which has been employed to constitute the positive electrode is a costly material as compared with other oxides of nickel, manganese and iron. Therefore, alternative employment of a relatively low-cost metal oxide has been required. In particular, employment of a manganese oxide which is one of transition metal materials exhibiting lowest cost to constitute the positive electrode has been required. As a representative manganese oxide, a spine compound LiMn2O4 is known. The theoretical capacity of the foregoing compound is smaller than 150 mAh/g which is smaller than 274 mAh/g of LiCoO2. The reason for this lies in that LiMn2O4 contains Li atoms, the number of which is half of Li atoms in LiCoO2 which include Li atoms by the same number as those in transition metal.
Therefore, LiMnO2 having a theoretical capacity equivalent to that of LiCoO2 has energetically been studied as a candidate of the positive-electrode active material containing manganese. According to reports of studies in the early stage, high-temperature LiMnO2 and low-temperature LiMnO2 have been reported.
High-temperature LiMnO2 has first been reported by W. D. Johnston et al. (J. Am. Chem. Soc., 78, 325 (1956)). Then, R. Hoppe, G. Brachtel and M. Jansen (Z. Anorg. Allg. Chmie, 417, 1 (1975) have determined the structure. Low-temperature LiMnO2 has first been reported by T. Ozuka, A. Ueda and T. Hirai (Chem. Express, Vol. 7, No. 3, 193 (1992)).
Each of high-temperature LiMnO2 and low-temperature LiMnO2 has a structure incorporating orthorombic lattices and defined by space group Pmnm. The theoretical capacity of each of the high-temperature LiMnO, and low-temperature LiMnO2 is about 300 mAh/g. The theoretical capacity cannot be realized when the charge/discharge conditions which are adapted to the present non-aqueous electrolyte batteries are employed.
According to the foregoing documents, the charge capacity of high-temperature LiMnO2 is 150 mAh/g and that of the low-temperature LiMnO2 is 200 mAh/g. The initial discharge capacity of high-temperature LiMnO2 is not higher than 50 mAh/g (2.0 Vxe2x89xa7V (Li/Li+), while that of low-temperature LiMnO, is 190 mAh/g (2.0 Vxe2x89xa7V (Li/Li+)).
The foregoing values are those measured when the current density is 100 xcexcA/cm2 or lower. To practically employ the foregoing manganese oxides, the foregoing values must be realized when the current density is 500 xcexcA/cm2 or higher. When the foregoing manganese oxide is used at the high-load current density, the discharge capacities of the high-temperature LiMnO2 and the low-temperature LiMnO2 are reduced to about 40 mAh/g and about 120 mAh/g, respectively.
The foregoing phenomenon are caused from the following two factors. That is, each material has a crystralline structure as shown in FIG. 1 that sheets each of which is constituted by Mnxe2x80x94O are laminated such that Li is introduced between the Mnxe2x80x94O sheets. The foregoing crystalline structure, however, has the diffusion paths for Li formed into a zigzag shape. Thus, quick diffusion of Li cannot be observed. Another reason lies in that the high-temperature LiMnO2 has high crsytallinity. Therefore, the electron resistance caused from impurity failure is low. On the other hand, low-temperature LiMnO2 having low crystallinity encounters high resistance caused from impurity. Therefore, low-temperature LiMnO2 encounters considerable IR drop owing to a high load, that is, a high current density. Therefore, great energy loss occurs. Under the foregoing circumstances, LiMnO2 having a flat layer structure permitting quick diffusion of Li and exhibiting high crystallinity has been required.
In 1996, Armstrong et al. has prepared LiMnO2, having space symmetry of C2/m by ion-substituting Na of NaMnO2 (A. R. Armstrong et aluminum., Nature, 381,499 (1996)). Thus, a first report about LiMnO2 having the flat structure has been made. Then, in 1998, a report has been made that the same structure as that of LiMnO2 reported by Armstrong et al. can be obtained by performing preparation of LiMn0.75.,Al0.25O2 such that partial pressure of oxygen is controlled (Y. Jang et aluminum., Electrochemical and Solid-State Letters, 1, (1) 13 (1998)). The reported compound, which has been prepared at high temperatures, has high crystallinity. Moreover, enlargement of the capacity under a high load has been expected owing to the flat diffusion paths for Li.
Each LiMnO2 encounters change in the structure (formed into a spinel structure) during the charging process, causing the discharge capacity to be reduced. In particular, LiMnO2 of the type reported by Armstrong et al. encounters considerable change in the structure during the charging process. Thus, a satisfactorily large discharge capacity cannot be obtained.
On the other hand, a consideration is made that LiMn0.75Al0.25O2 is not thermodynamically unstable as compared with LiMnO2. As compared with LiMnO2, change in the structure does not considerably occur. However, formation of a solid solution of Al which is electrochemically inactive results in reduction in the capacity.
In view of the foregoing, an object of the present invention to provide a non-aqueous electrolyte battery free from considerable change in the structure of a positive-electrode active material thereof and capable of enlarging the capacity thereof.
To achieve the object, according to one aspect of the present invention, there is provided a non-aqueous electrolyte battery comprising: a positive electrode containing a positive-electrode active material; a negative electrode containing a negative-electrode active material to which Li can be doped/dedoped; and a non-aqueous electrolyte disposed between the positive electrode and the negative electrode and containing non-aqueous solvent and an electrolyte, wherein a material expressed by general formula LiMn1xe2x88x92yAlyO2, (0.06xe2x89xa6y less than 0.25) is contained as the positive-electrode active material and LiMn1xe2x88x92yA1xe2x88x92yO2 has a crystalline structure expressed by space group C2/m.
The non-aqueous electrolyte battery according to the present invention and incorporating a layered compound expressed by LiMn1xe2x88x92yAlyO2 is able to enhance diffusion of lithium ions. Since the non-aqueous electrolyte battery according to the present invention is structured such that the value of y of LiMn1xe2x88x92yAlyO2 is specified, deterioration in the electric conductivity can be prevented. Moreover, thermal stability of the crystalline structure can be improved.